Electrodes with multicomponent coatings

ABSTRACT

Electrodes useful in a wide variety of electrolytic processes comprise a conductive substrate bearing on at least a portion of the surface thereof a four-component coating, said components being the oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum.

Franks et al.

Electronor Corporation, Panar City, Panama Filed: Apr. 19, 1973 Appl.No.: 352,499

Assignee:

U.S. Cl 204/290 F, 204/98 Int. Cl. B01K 3/04 Field of Search 204/290 F,95

References Cited UNITED STATES PATENTS 9/1966 Messner 204/219 10/1971Bianchi et al. 204/290 F 1 Apr. 1, 1975 3,627,669 12/1971 Entwisle eta]. 204/290 F 3,684,543 8/1972 DeNora et a1 204/290 F 3,725,223 4/1973DeNora et al. 204/99 3,732,157 5/1973 Dewitt 204/95 3,751,296 8/1973Beer 204/290 F 3,776,834 12/1973 OLeary 204/96 3,779,889 12/1973Loftfield 204/95 3,793,164 2/1974 Kolb et al. 204/99 Primary ExaminerO.R. Vertiz Assistant Examiner-Wayne A. Langel Attorney, Agent, orFirml-lammond & Littell [57] ABSTRACT Electrodes useful in a widevariety of electrolytic processes comprise a conductive substratebearing on at least a portion of the surface. thereof a fourcomponentcoating, said components being the oxides of tin, antimony, at least oneplatinum group metal, and a valve metal selected from the group titaniumand tantalum.

14 Claims, No Drawings ELECTRODES WITH MULTICOMPONENT COATINGSBACKGROUND OF THE INVENTION Recent years have seen a proliferation ofdimensionally stable electrodes, i.e., wear resistant conductivesubstrates bearing on the surface thereof an electrically conductive,electrocatalytically active coating. Among these have been electrodescoated with (1) an antimony oxide-doped tin oxide containing a platinummetal oxide as the electrocatalytic agent or (2) mixed crystals (solidsolutions) of a valve metal oxide and a platinum metal oxide. While suchelectrodes are far superior to the previously employed graphite,particularly in the area of chlor-alkali electrolysis, understandableefforts have continued to extend the life of these electrodes (that is,reduce the platinum metal wear-rate per unit of product) and/or toreduce the tendency of the coatings to passivate (that is, increase inoperating potential to a point at which further operation becomesimpractical), especially under oxygen-evolving conditions. Further,owing to inherent limitations relating both to life and passivationtendencies, no single electrode coating system has been found applicableto use in a wide variety of electrochemical processes.

STATEMENT OF THE INVENTION Therefore, it is an object of the presentinvention to provide a coated electrode having a long coating life.

It is a further object of the present invention to provide an electrode,the coating of which is extremely resistant to passivation.

It is a still further object of the present invention to provide anelectrode, the properties of which may be adapted for use in a varietyof electrochemical processes.

These and further objects of the present invention will become apparentto those skilled in the art from the specification and claims thatfollow.

There has now been found an electrode comprising an electricallyconductive supporting substrate bearing on at least a portion of thesurface thereof a coating consisting essentially of from L to 10.0percent antimony oxide, from 30 to 90 percent tin dioxide, from 1.0 to50 percent of at least one platinum group metal oxide, and from 0.5 to30 percent of a valve metal oxide selected from the group consisting oftitanium and tantalum oxides, with the proviso that the mole ratio oftin to antimony oxides is between 95:5 and 85:15. Further, within theaforestated ranges, those coatings having high valve metal and platinummetal oxide concentrations are particularly useful as anodes at whichoxygen is evolved. On the other hand, those coating compositions havinglow valve metal oxide concentrations and moderate concentrations ofplatinum metal oxides are particularly useful in chlor-alkalielectrolysis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As stated, the invention liesin a combined coating of oxides of tin, antimony, at least one platinumgroup metal, and a valve metal selected from the group titanium andtantalum on a conductive substrate, useful as an electrode, especiallyas-an anode, in a variety of electrochemical processes includingelectrowinning of metals (e.g., copper, nickel, and zinc) from aqueoussolution; chlor-alkali electrolysis including chlorine,

chlorate, or hypochlorite production; electroplating;

oxygen evolution from organic acidic solutions; ozone generation;cathodic protection; electrodialysis; and 5 the like.

Suitable substrates include generally any metal of sufficient electricalconductivity and mechanical and chemical resistance to the cellenvironment in which it is to be employed. For example, these materialsmay include nickel, steel, stainless steel, titanium, niobium,zirconium, and tantalum. Especially preferred for most applications aretitanium, niobium, or tantalum substrates. Of course, those substratesbearing an exterior coating, such as copper or aluminum-cored titaniumor a platinum or other conductive metal layer over a titanium substrate,are contemplated. Generally, prior to deposition of the coating and inorder to provide a base to which the coating may be satisfactorilyanchored, an etching or other cleaning operation is employed.

The configuration of the electrode will vary considerably with theapplication intended but may generally be in the form of a rod or asheet, either continuous or foraminous, of the appropriate material.

What may be considered the first of the components in the coatingcomposition is tin dioxide, preferably present in the form ofcrystalline SnO and employed within the range of from 30 to 90 percentby weight of the total coating composition on an oxide basis, especially30 to 50 percent for oxygen applications and 60 to 90 percent forchlorine.

The antimony oxide component enters into the tin oxide crystal lattice,rendering same more electrically conductive. Although the antimony ispresent in an indeterminate oxide form owing to its entrance into thetin oxide crystal lattice, it may be expressed for convenience sake asSb O Thus, on this basis, the antimony oxide is present within the rangeof from 1.0 to 10, preferably 4.0 to l0, percent by weight.

The foregoing ranges of tin and antimony oxides are further qualified bythe proviso that they be present, respectively, in the range, on a moleratio basis as the oxides, of 95:5 to 85:15, especially 90:10. In thisfashion there is obtained the desired doping effect of the antimony onthe tin oxide without the presence of an excess separate phase ofantimony oxides.

The third component of the coating is at least one "platinum group metaloxide, by which term it is intended to include the oxides of platinum,palladium, ruthenium, iridium, rhodium, and osmium, preferablyruthenium, iridium, rhodium, and palladium, and especially mixtures ofruthenium with iridium, rhodium, or palladium oxides/These platinumgroup metal oxides are present in their most highly oxidized form andwithin the range of from 1.0 to 50 percent by weight. When the electrodeis being fabricated for use as an anode at which oxygen is evolved,primarily or as a co- Ta O it is understood that mixtures of tantalumoxides may in fact be present. The amounts of valve metal oxidesemployed are generally within the range of from 0.5 to 30 percent byweight, especially 15 to 25, for

trichloride or pentachloride, and stannic chloride or dibutyl tindichloride.

It will be understood by those skilled in the art that it is possible touse a number of combinations of preoxygen-evolving applications and 0.5to 3.0 for chlor- 5 formed oxides of the various component metals andalkali electrolysis. Further, for a chlor-alkali applicasalts of theremaining materials, although it is generally tion, titanium ispreferred as the valve metal whereas believed that preformed valve metaloxides should not in oxygen-evolving applications the preferred valve beemployed nor should separately preformed tin and metal is tantalum,although they are interchangeable in antimony oxides be used. Further,if thermal decompomany instances. Generally speaking, the use of small lsition is incomplete, small amounts of salts may remain amounts of thevalve metal oxide acts to extend the life without detrimental effect inthe coating, for example, of the electrode coating while theincorporation of small amounts of chloride in the primarily oxidecoatlarger amounts adds resistance to passivation. g-

In summary, an example of a preferred anode for In order that thoseskilled in the art may more readily oxygen-evolving applications is acoating of from 30 t 15 understand the present invention and certainpreferred er t S O 4 O t 8,0 r t 51 0 20 to 40 embodiments by which itmay be carried into effect, percent platinum metal oxide, and to 25percent following Specific examples are afforded valve metal oxide on atitanium, tantalum, or niobium substrate. EXAMPLE 1 On the other hand,an example of a preferred chlo- A series of electrodes is prepared andevaluated as fine anode is a Coating 0f 60 i0 90 Percenl z, 4 t0 anodesas follows. In each instance, the quantity of 10 percent Sb O l .0 topercent platinum metal dithermally decomposable salt set forth in Tablel is disoxide, and 0.5 to 3.0 percent titanium or tantalum solved in 45ml of ethanol with stirring. The resultant oxide on a titaniumsubstrate. 25 solution is brushed onto an expanded titanium mesh Whilemany of the variety of methods known for prosubstrate, previouslycleaned by etching for minutes ducing mixed metal oxide coatings may beemployed, in boiling (l8%) aqueous hydrochloric acid. The soluthepreferred method of preparing the multicomponent tion is applied to themesh by brushing, followed by coating composition of the substrate is bydeposition drying the anode for 3 minutes at 1 10 C and firing in from asolution of the appropriate thermochemically 30 air at 500 C and 7minutes. This brushing, drying, and decomposable salts. For example, itis desirable to baking procedure is repeated untilacoating containingpaint or brush an acidified alcoholic solution of said 1.7 grams ofruthenium per square foot of anode sursalts onto the substrate followedby drying at l00l40 face is obtained (usually 6-10 coats). Following theC for from 3 to 10, especially 5, minutes and finally by final baking,the electrodes are evaluated as anodes in baking in an oxidizingatmosphere, e.g., air, at 450 to 3 a 150 g/l sulfuric acid solution at 3amperes per square 520 C, espically 500 C, for from 5 to l0, especiallyinch opposite a titanium mesh cathode and at an elecabout 7, minutes.This procedure may then be repeated trode gap of 2 inches. The test iscontinued until the anany number of times until the desired coatingthickness odes have passivated, i.e., a voltage of 8.0 volts or isobtained, for example, 6 to 10 coats. The preferred greater is obtained.The lifetime of the anode, that is, solvents for the thermallydecomposable salts are the the number of hours of successful operationuntil paslower alkanols, such as ethanol, propanol, amyl alcosivationoccurs, is reported in the following Table 1.

TABLE I RuCl .xH O SnCl .5H O sno sbcl sb o, (38%) R60 Tact, Ta O,,Lifetime Anode g 71 g 71 g 7! g '4 hrs.

hol, and especially n-butyl alcohol, although other sol- From this it isapparent that Anodes 4 and 5, accordvents including water, may beemployed, to which ing to the present invention, are greatly superior toeithere is generally added from 0 to percent by volther an anodecombining the valve metal and platinum ume of an acid, such asconcentrated hydrochloric acid group metal (anode l) or the platinummetal- (36%). The concentration of the salts from which the y- System(Anode Further, Anode 3illl1S- coating composition is derived is such asto give a metal trates that the range of components of the presentincontent in solution within the range of 50 to 200 grams vention iscritical to obtaining an anode having a long per liter. The saltsemployed are generally any ther- ,0 lif mally decomposable inorganic ororganic salt or organic ester of the metals in question such as thechlo- EXAMPLE 2 rides, nitrates, alkoxides, alkoxy halides, resinates,Four electrodes were prepared from the f ll i amines, and the like.Specific and illustrative examples l i include potassiumhexachlororuthenate, hexachloroi-. Anode 6 50 ml n-butanol, 12.5 g SnCl-5H O, 0.9

ridic acid, ruthenium trichloride or tribromide, or-' thobutyl titanate,tantalum pentachloride, antimony g SbCl and 1.] g RuClg-xH o (38% Ru).Anode 7 45 ml ethanol, 5.0 g orthobutyl titanate,

1.1 g SbCl 15.1 g SnCl -5H O, and 7.6 g RuCl -x- H (38% Ru).

Anode 8 50 ml n-butanol, 12.5 g SnCL'SH O, 0.91 g SbCl 7.0 g orthobutyltitanate, and 1.1 g RuCl -xl-l O (38% Ru).

Anode 9 45 ml ethanol, 4.5 g TaCl,=,, 1.1 g SbCl 15.1 g SnCl -5H O, and7.6 g RuCl -xH O (38% Ru).

Each anode is prepared by applying six coats of the solution by brush,with heating in air between each coat first at 1 C for 3 minutesfollowed by 7 minutes at 500 C.

These electrodes are evaluated as anodes in a horizontal mercury cellspaced 0.14 inch above and parallel to a mercury cathode flowing at arate of 450 ml/minute. The electrolyte is a 310 g/l brine solutionhaving a pH within the range of 3-6 and a temperature of about 70 C. Toestablish the wear-rate of the anodes, electrolysis is conducted at 6amperes per square inch for 500 hours, the loss being determined byweight differential. Results, together with the composition of eachanode coating calculated on an oxide basis, appear in Table 2.

weight basis, from to 90 percent SnO from 1.0 to 50 percent of at leastone platinum group metal oxide, and from 0.5 to 30 percent of a valvemetal oxide selected from the group consisting of titanium and tanta lumoxides, with the proviso that the mole ratio of tin to antimony oxidesis between 95:5 and 85:15.

2. An electrode as in claim 1 wherein the supporting substrate isselected from the group consisting of nickel, steel, stainless steel,titanium, niobium, zirconium, and tantalum.

3. An electrode as in claim 1 wherein the platinum metal oxide is RuO 4.An electrode as in claim 1 wherein the valve metal oxide is TiO 5. Anelectrode as in claim 1 wherein the valve metal oxide is amorphoustantalum oxide.

6. An electrode as in claim 1 wherein the ratio of tin to antimonyoxides is about 90:10.

7. An anode for use in oxygen-evolving applications, which anodecomprises an electrically conductive supporting substrate bearing on atleast a portion of the surface thereof a coating consisting essentiallyof from 4.0 to 8.0 percent antimony oxide, calculated as Sb O AnodeWear-Rate g/ton C 1 EXAMPLE 3 An anode coating solution if prepared from45 ml ethanol, 4.5 g TaCI 1.1 g SbCl 15.1 g SnCl -5H O, and 7.6 g RuCl-xH O (38% Ru). An etched titanium mesh substrate is coated by brushing,drying at 1 10 C for 3 minutes, and baking in air at 500 C for 7minutes. The coating procedure is repeated until a coating having aplatinum group metal content of 1 gram per square foot is obtained. Thisis labeled Anode l0.

Anode l l is prepared in an identical fashion but substituting 0.92 g oflrCl and 6.54 g RuCl 'xH O for the ruthenium content of Anode l0. Anode12 is likewise similar with the exception that 1.28 g of RhCl -3H O and6.65 g RuCl -xH O comprise the platinum group metal content.

When evaluated according to the lifetime test described in Example 1above, Anodes l0, l1, and 12 have lifetimes, respectively, of 185, 250,and 350 hours. This indicates the substantial improvement possibleemploying a mixture of platinum metal oxides in the coating.

We claim:

1. An electrode comprising an electrically conductive supportingsubstrate bearing on at least a portion of the surface thereof a coatingconsisting essentially of from 1.0 to 10 percent antimony oxide, as Sb Oon a on a weight basis, from 30 to 50 percent SnO from 20 to 40 percentof at least one platinum metal oxide, and from 15 to 25 percent of avalve metal oxide selected from the group consisting of titanium andtantalum oxides, with the proviso that the mole ratio of tin to antimonyoxides is between 95:5 and :15.

8. An anode as in claim 7 wherein the substrate is selected from thegroup consisting of nickel, steel, stainless steel, titanium, niobium,zirconium, and tantalum.

9. An anode as in claim 7 wherein the platinum metal oxide is acombination of RuO and lrO 10. An anode as in claim 7 wherein theplatinum metal oxide is a combination of ruthenium and rhodium oxides.

11. An anode as in claim 7 wherein the valve metal oxide is amorphoustantalum oxide.

12. An anode for use in chlor-alkali electrolysis, which anode comprisesa valve metal substrate selected from the group consisting of titanium,niobium, zirconium, and tantalum bearing on at least a portion of thesurface thereof a coating consisting essentially of from 4.0 to 10percent antimony oxide, calculated as Sb O on a weight basis, from 60 topercent SnO from 1.0 to 25 percent of at least one platinum group metaloxide, and from 0.5 to 3.0 percent of a valve metal oxide selected fromthe group consisting of titanium and tan-' talum oxides, with theproviso that the mole ratio of tin to antimony oxides is between :5 and85:15.

13. An anode as in claim 12 wherein the substrate is titanium.

14. An anode as in claim 12 wherein the valve metal oxide is TiO

1. AN ELECTRODE COMPRISING AN ELECTRICALLY CONDUCTIVE SUPPORTINGSUBSTRATE BEARING ON AT LEAST A PORTION OF THE SURFACE THEREOF A COATINGCONSISTING ESSENTIALLY OF FROM 1.0 TO 10 PERCENT ANTIMONY OXIDE, ASSB2O3, ON A WEIGHT BASIS, FROM 30 TO 90 PERCENT SNO2, FROM 1.0 TO 50PERCENT OF AT LEAST ONE PLATINUM GROUP METAL OXIDE, AND FROM THE GROUPCONSISTING OF TITANIUM METAL OXIDE SELECTED FROM THE GROUP CONSISTING OFTITANIUM AND TANTALUM OXIDES, WITH THE PROVISO THAT THE MOLE RATIO OFTIN TO ANTIMONY OXIDES IS BETWEEN 95.5 AND 85.15.
 2. An electrode as inclaim 1 wherein the supporting substrate is selected from the groupconsisting of nickel, steel, stainless steel, titanium, niobium,zirconium, and tantalum.
 3. An electrode as in claim 1 wherein theplatinum metal oxide is RuO2.
 4. An electrode as in claim 1 wherein thevalve metal oxide is TiO2.
 5. An electrode as in claim 1 wherein thevalve metal oxide is amorphous tantalum oxide.
 6. An electrode as inclaim 1 wherein the ratio of tin to antimony oxides is about 90:10. 7.An anode for use in oxygen-evolving applications, which anode comprisesan electrically conductive supporting substrate bearing on at least aportion of the surface thereof a coating consisting essentially of from4.0 to 8.0 percent antimony oxide, calculated as Sb2O3, on a weightbasis, from 30 to 50 percent SnO2, from 20 to 40 percent of at least oneplatinum metal oxide, and from 15 to 25 percent of a valve metal oxideselected from the group consisting of titanium and tantalum oxides, withthe proviso that the mole ratio of tin to antimony oxides is between95:5 and 85:
 15. 8. An anode as in claim 7 wherein the substrate isselected from the group consisting of nickel, steel, stainless steel,titanium, niobium, zirconium, and tantalum.
 9. An anode as in claim 7wherein the platinum metal oxide is a combination of RuO2 and IrO2. 10.An anode as in claim 7 wherein the platinum metal oxide is a combinationof ruthenium and rhodium oxides.
 11. An anode as in claim 7 wherein thevalve metal oxide is amorphous tantalum oxide.
 12. An anode for use inchlor-alkali electrolysis, which anode comprises a valve metal substrateselected from the group consisting of titanium, niobium, zirconium, andtantalum bearing on at least a portion of the surface thereof a coatingconsisting essentially of from 4.0 to 10 percent antimony oxide,calculated as Sb2O3, on a weight basis, from 60 to 90 percent SnO2, from1.0 to 25 percent of at least one platinum group metal oxide, and from0.5 to 3.0 percent of a valve metal oxide selected from the groupconsisting of titanium and tantalum oxides, with the proviso that themole ratio of tin to antimony oxides is between 95:5 and 85:15.
 13. Ananode as in claim 12 wherein the substrate is titanium.
 14. An anode asin claim 12 wherein the valve metal oxide is TiO2.